In order to realize a high-capacity optical communication, multiplexing of optical signals performed by wavelength division multiplexing (WDM) is widespread. In the WDM transmission system, a WDM signal in which a plurality of wavelength channels are multiplexed is transmitted. A reconfigurable optical add drop multiplexer (ROADM) is provided in each node of the WDM transmission system as an optical transmission device. The ROADM may split an optical signal of a desirable wavelength channel from the WDM signal and add the optical signal to an empty channel of the WDM signal.
The ROADM includes a pre-amplifier, a post-wavelength selective switch (WSS), a post-wavelength selective switch, a multiplexer (MUX)/demultiplexer (DMUX), a plurality of transponders, a post-amplifier, and the like.
The ROADM adjusts an output power of the amplifier and an output power of the WSS such that a WDM signal which is input becomes a target power. For example, the total power at an output terminal of the amplifier is monitored by a photo detector (PD), and the output power of the amplifier is continually feedback-controlled so as to reach a target power. The power of each wavelength at an output terminal of the WSS is monitored by an optical channel monitor (OCM), and the amount of attenuation of the WSS is continually feedback-controlled so as to reach the target power.
In the related art, a technique for controlling power of a signal output from an optical cross-connection device to be steady is disclosed. A technique for detecting a shift in wavelength transmission characteristics of a wavelength multiplexing filter by using the OCM is disclosed.
Recently, Interconnection of optical transmission devices to other companies is proposed. In a case where the optical transmission device is connected to optical transmission devices of other companies different from the own device, a format of an optical supervisory channel (OSC) and content of information are different from each other for each company, and thus, there is a limitation on transmission of control information. Accordingly, in a case of a connection between the own devices, the control information of adjacent devices, which is acquired via the OSC, for example, the number of wavelengths, a wavelength disposition, CH power of an input and an output of the WSS, attenuation (AU) of the WSS, may not be acquired from the devices of other companies.
In this case, if an output power steady control of the related art is performed, for example, in a case where the number of wavelengths varies, if each device performs an output power control at the same time, large overshoot and undershoot occur, and there is a danger of damage of the device. In order to avoid this, a method of controlling a gain of the device to be steady is considered. However, in a case of the gain steady control, only total power monitor performed by the PD may be acquired from an input and an output of the device. A gain of the optical amplifier has wavelength dependence. Accordingly, the gains of each CH may not be correctly monitored, and the gain steady control may not be performed with a high accuracy.
As such, in a case where an optical transmission device is connected to optical transmission devices of other companies, the optical transmission device such as a ROADM may not correctly monitor a gain between an input and an output, and despite the fact that the gain does not reach a target gain in actual, it is erroneously recognized that a gain reaches the target gain. In the WDM transmission system in which the ROADMs are connected in multiple stages, power deviation in each ROADM is accumulated and a signal quality deteriorates.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 2011-254309 and
[Document 2] Japanese Laid-open Patent Publication No. 2014-007564.